Technical Field
The present disclosure relates to a toner for use in electrophotography, electrostatic recording, electrostatic printing, etc., and a developer using the toner.
Description of the Related Art
In electrophotography, electrostatic recording, electrostatic printing, etc., a toner is once adhered to a latent image bearer, such as an electrostatic latent image bearer, on which an electrostatic latent image has been formed in a process called developing process. The toner is then transferred from the electrostatic latent image bearer onto a transfer medium such as paper in a process called transfer process. The toner is then fixed on the transfer medium in a process called fixing process. In the fixing process, the toner is generally melted by contact with a heated roller or belt, which is an advantageous method in terms of thermal efficiency. (This method is hereinafter referred to as contact heating fixing method.)
However, the contact heating fixing method is likely to cause an offset phenomenon in which toner is disadvantageously adhered to the heated roller or belt.
In attempting to prevent the occurrence of the offset phenomenon, toners containing a release agent, such as a wax, have been proposed. For example, a toner containing a wax having a predetermined endothermic peak observable by differential scanning calorimetry has been proposed. As another example, a toner containing a candelilla wax, a higher fatty acid wax, a higher alcohol wax, a plant natural wax (e.g., carnauba wax, rice wax), or a montan wax has been proposed.
In the contact heating fixing method, the release agent is melted in a rapid manner when the toner passes through the heated roller or belt. The release agent is thereby exposed at the surface of the toner. The exposed release agent prevents the toner from adhering to the fixing member (i.e., the roller or belt). The release agent exerts influence on the occurrence of both a cold offset phenomenon, which is occurred at low temperatures, and a hot offset phenomenon, which is occurred at high temperatures.
In a case in which the release agent is positioned near the surface of the toner for the purpose of accelerating exposure of the release agent, the occurrence of the offset phenomenon can be prevented, but the release agent will have come to adhere to other members while the toner is being stirred in a developing device. The toner will be pressed against carrier particles or photoconductors and firmly adhere thereto, causing decrease in charge quantity. The release agent should be protected inside the toner when the toner is being stirred or stored. On the other hand, the release agent should be efficiently exposed at the surface of the toner in such a short time during which the toner passes through the fixing member.
Many attempts have been made to determine a proper dispersion particle diameter for the release agent dispersed in the toner for preventing the occurrence of the offset problem while maintaining toner productivity. It is generally very difficult to contain the wax in the form of fine particles inside the toner without exposing them at the surface of the toner because the wax particles are inevitably finer than the toner particles.
From the standpoint of giving resistance to the offset phenomenon (hereinafter “hot offset resistance”) to the toner, it is more effective that the release agent exists in the form of a relatively large block rather than in the form of fine particles locally distributed over the toner. If the large block of the release agent is achieved by excessively increasing the content of the release agent, the toner will deteriorate in strength and become easy to get crushed, causing charge quantity deterioration and background fouling.
A toner containing a release agent having predetermined aspect ratio and size has been proposed. The toner is reported to have improved low-temperature fixability, back fouling resistance, and chargeability. However, this toner is considered to be still insufficient in terms of durability or strength while maintaining offset resistance and toner chargeability. In particular, when the toner is used for a non-magnetic one-component developing process, the toner will be excessively loaded with a toner layer regulating blade and firmly fixed thereto, causing deterioration in image quality. Therefore, the toner is required to have durability higher than that of two-component developers.
Accordingly, there has been a demand for a toner which achieves a good combination of offset resistance and durability with a small amount of release agent.